This article addresses the issue of peer effects on a swimmer’s performance. The National Database of Student Athletes in Taiwan from 2008 to 2010 is employed. The results show that a swimmer performs better when his or her competitors are faster. The evidence shows that peer effects are positive. As to the relative quality of swimmers in a competition, dispersed-quality competitors make a swimmer faster. The evidence also shows that older and taller boys swim faster. The regressions of the Heckman selection model support these conclusions.